Liquid Crystal Display Device

1. A liquid crystal display device comprising: a plurality of light sources emitting a plurality of lights; and a pixel portion, the pixel portion comprising: a first pixel electrode having a light-transmitting property; and a second pixel electrode which is a reflective electrode and placed adjacently to the first pixel electrode, wherein the first pixel electrode is electrically connected to a first transistor and the second pixel electrode is electrically connected to a second transistor, wherein the pixel portion is divided into a plurality of regions, wherein color display is performed by controlling the plurality of lights whose hues are different per the plurality of regions and applying a voltage to a first region of the liquid crystal layer overlapping with the first pixel electrode, wherein a monochrome still image is displayed by driving the second transistor in a period when the plurality of light sources are turned off and applying a voltage to a second region of the liquid crystal layer overlapping with the second pixel electrode, wherein the color display is a moving image, wherein an interval of writings of the monochrome still image is longer than that of the color display, wherein the reflective electrode of an adjacent pixel is above the first transistor, and wherein the reflective electrode of the adjacent pixel overlaps with an active layer of the first transistor.

2. The liquid crystal display device according to claim 1 , wherein the first transistor and the second transistor comprise an oxide semiconductor.

3. The liquid crystal display device according to claim 2 , wherein the oxide semiconductor is an In—Ga—Zn—O-base oxide semiconductor.

4. The liquid crystal display device according to claim 2 , wherein a hydrogen concentration of the oxide semiconductor is less than or equal to 5×10 19 /cm 3 .

5. The liquid crystal display device according to claim 1 , wherein an off-state current density of the first transistor and the second transistor is less than or equal to 100 yA/μm.

6. A liquid crystal display device comprising: a plurality of light sources emitting a plurality of lights whose hues are different from each other; and a pixel portion, the pixel portion comprising: a first pixel electrode having a light-transmitting property; and a second pixel electrode which is a reflective electrode and placed adjacently to the first pixel electrode, wherein the first pixel electrode is electrically connected to a first transistor and the second pixel electrode is electrically connected to a second transistor, wherein the pixel portion comprises a first region and a second region, wherein color display is performed by controlling the plurality of lights whose hues are different per the first region and the second region and applying a voltage to a first region of the liquid crystal layer overlapping with the first pixel electrode, wherein a monochrome still image is displayed by driving the second transistor in a period when the plurality of light sources are turned off and applying a voltage to a second region of the liquid crystal layer overlapping with the second pixel electrode, wherein the color display is a moving image, wherein an interval of writings of the monochrome still image is longer than that of the color display, wherein the reflective electrode of an adjacent pixel is above the first transistor, and wherein the reflective electrode of the adjacent pixel overlaps with an active layer of the first transistor.

7. The liquid crystal display device according to claim 6 , wherein the first transistor and the second transistor comprise an oxide semiconductor.

8. The liquid crystal display device according to claim 7 , wherein the oxide semiconductor is an In—Ga—Zn—O-base oxide semiconductor.

9. The liquid crystal display device according to claim 7 , wherein a hydrogen concentration of the oxide semiconductor is less than or equal to 5×10 19 /cm 3 .

10. The liquid crystal display device according to claim 6 , wherein an off-state current density of the first transistor and the second transistor is less than or equal to 100 yA/μm.

11. A liquid crystal display device comprising: a plurality of light sources emitting a plurality of lights whose hues are different from each other; and a pixel portion, the pixel portion comprising: a first pixel electrode having a light-transmitting property; and a second pixel electrode which is a reflective electrode and placed adjacently to the first pixel electrode, wherein the first pixel electrode is electrically connected to a first transistor and the second pixel electrode is electrically connected to a second transistor, wherein the pixel portion comprises a first region and a second region, wherein color display is performed by sequentially supplying the plurality of lights to the first region in a first rotating order and to the second region in a second rotating order and applying a voltage to a first region of the liquid crystal layer overlapping with the first pixel electrode, wherein a monochrome still image is displayed by driving the second transistor in a period when the plurality of light sources are turned off and applying a voltage to a second region of the liquid crystal layer overlapping with the second pixel electrode, wherein the first rotating order is different from the second rotating order, wherein the color display is a moving image, wherein an interval of writings of the monochrome still image is longer than that of the color display, wherein the reflective electrode of an adjacent pixel is above the first transistor, and wherein the reflective electrode of the adjacent pixel overlaps with an active layer of the first transistor.

12. The liquid crystal display device according to claim 11 , wherein the first transistor and the second transistor comprise an oxide semiconductor.

13. The liquid crystal display device according to claim 12 , wherein the oxide semiconductor is an In—Ga—Zn—O-base oxide semiconductor.

14. The liquid crystal display device according to claim 12 , wherein a hydrogen concentration of the oxide semiconductor is less than or equal to 5×10 19 /cm 3 .

15. The liquid crystal display device according to claim 11 , wherein an off-state current density of the first transistor and the second transistor is less than or equal to 100 yA/μm.

16. The liquid crystal display device according to claim 1 , wherein an interval between writings of image signals is 10 seconds or more when the monochrome still image is displayed.

17. The liquid crystal display device according to claim 6 , wherein an interval between writings of image signals is 10 seconds or more when the monochrome still image is displayed.

18. The liquid crystal display device according to claim 11 , wherein an interval between writings of image signals is 10 seconds or more when the monochrome still image is displayed.